Motivated by questions in robust control and switched linear dynamical systems, we consider the problem checking whether all convex combinations of k matrices in Rntimesn are stable. In particular, we are interested whether there exist algorithms which can solve this problem in time polynomial in n and k. We show that if k=nd for any fixed real d > 0, then the problem is NP-hard, meaning that no polynomial-time algorithm in n exists provided that P ne NP, a widely believed conjecture in computer science. On the other hand, when k is a constant independent of n, then it is known that the problem may be solved in polynomial time in n. Using these results and the method of measurable switching rules, we prove our main statement: verifying the absolute asymptotic stability of a continuous-time switched linear system with more than nd matrices Ai isin Rntimesn satisfying 0 ges Ai + AiT is NP-hard.